[LibTooling] Add Transformer, a library for source-to-source transformations.
Summary: Adds a basic version of Transformer, a library supporting the concise specification of clang-based source-to-source transformations. A full discussion of the end goal can be found on the cfe-dev list with subject "[RFC] Easier source-to-source transformations with clang tooling".
Reviewed By: ilya-biryukov
Subscribers: ioeric, ABataev, mgorny, jfb, jdoerfert, cfe-commits
Differential Revision: https://reviews.llvm.org/D59376